Electrolytic system and method for filtering an aqueous particulate suspension

ABSTRACT

An apparatus for the concentration of suspended algae particles in an aqueous solution. The apparatus includes an electrolytic cell containing at least an anode and a cathode, and a filter. The electrolytic cell receives a solution containing suspended algae particles therein. A power supply is near the filter. A zone of depleted suspended algae particles is near the filter, formed under the influence of an applied electric field from the power supply.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. application Ser. No.13/249,322, filed Sep. 30, 2011, which in turn claims the benefit ofU.S. Provisional Application 61/392,738, filed Oct. 13, 2010, theentireties of which are both hereby incorporated by reference.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

This invention was made with U.S. Government support under NationalScience Foundation SBIR Grant No. IIP-0944523. The U.S. Government mayhave certain rights in the invention.

FIELD OF THE INVENTION

This invention relates to the filtration of aqueous solutions containingsuspended particles and, more particularly, to the filtration of aqueousgrowth mediums containing suspended algae. This invention furtherrelates to the filtration of such solutions under the influence of anelectric field, whereby clogging of the filter medium is minimized oreliminated.

BACKGROUND

Many industrially important processes require water removal apparatusand processes which consume low amounts of energy. More specifically,dewatering algae is a major hurdle in the economic production of algaederived biofuels, pharmaceuticals, and nutraceuticals.

Oil-producing microalgae are an attractive alternative to conventionalfuel sources as they grow extremely rapidly, have the potential toproduce as much as 100 times more oil per hectare of land area thanland-based crops, and, if properly exploited, can serve as a biofuelsource that is economical, sustainable, reduces global warming, reducesthe need to displace conventional food crops, and provides energyindependence.

Oleaginous microalgae use photosynthesis to capture carbon from the air(in the form of carbon dioxide) to produce the various cellularchemicals needed to live. One class of cellular chemicals of particularinterest is lipids. When properly grown, some of the oleaginousmicroalgae species can reach up to eighty percent lipid content (by celldry weight), which includes hydrocarbons (e.g., β-carotene, terpenoids),triglycerides, and other minor components (e.g., sterols, glycolipids,and phospholipids). While some of these hydrocarbons can often be useddirectly a diesel-grade motor fuel, many of these lipids, throughpost-processing, can also be transformed into other types of organiccompounds and feedstocks.

There are two main approaches to growing algae in bulk: closedbioreactors and open ponds. Bioreactors are often 10-100 times morecostly to build and operate as compared to open ponds, which limitstheir usefulness outside of products that require tight controls and arehighly value-added (e.g., pharmaceuticals and nutraceuticals). However,although cheaper, open ponds have some potential problems withcontamination and overall biomass concentration. One approach thatcombines some of the advantages of the bioreactor and open pond layoutsconsists of using large ponds covered with a canopy. This hybrid systemprotects the algae from many of the harmful effects of the environmentwhile still allowing for large, pond-like growth areas. However, a majorchallenge for production of algae derived biofuels still remains: theharvesting of the lipids from the algae in a large scale, cost-effectivemanner.

In order to use algae-derived hydrocarbons as a fuel or as a feedstockfor fuel production, pharmaceutical production, and/or nutraceuticalproduction, the algae must be separated from an aqueous growth medium(an aqueous solution containing trace elements such as nitrogen,phosphorous, etc.) using apparatuses and processes which requiresubstantially less energy input than the energy content of the algaebiomass or, alternately, less energy investment than the value of theproduct. For biofuel production, the target energy requirement, asstated by the U.S. Department of Energy, is that the energy must be lessthan or equal to 10% of the energy content of the biomass.

Several suspended particle separation technologies are known in the artand include centrifugation, flotation, filtration, sedimentation, andthe like. However, these either consume too much energy, operate at lowthroughput, or require the addition of chemicals that require subsequentremoval. Current methods to extract the oil from algae are solventextraction (for example, supercritical CO₂, hexane, benzene) and anexpeller press. Solvent extraction produces hazardous waste as abyproduct, and both solvent extraction and expeller press processes tendto be energy intensive, reducing the net energy yield of the oil.Solvent inputs and press machinery are also quite costly.

Filtration is a low energy consumption separation technology, andfiltration is often combined with other separation technologies in ahybrid concept. A significant limitation of filtration is that as theseparation process proceeds, the filter medium becomes clogged with thesuspended particles. At this point the separation of suspended particlesfrom the solution becomes slow, and the filter medium must be replaced,backwashed, or otherwise rejuvenated to re-establish acceptable solutionflow rates and/or differences in head. As the filter medium becomesclogged with suspended particles, the operating and capital cost offiltration-based separation technology becomes higher. Consequently,there is a need for filtration technology in which clogging of thefilter medium with suspended particles is minimized or eliminated.

SUMMARY OF THE INVENTION

The subject invention is an electrolytic apparatus and method for theseparation of suspended particles, such as algae, from an aqueoussolution, such as nitrogen and phosphorous enriched water. Theelectrolytic apparatus may direct an aqueous particulate suspensionbetween an electrode pair, and apply a current or voltage across theelectrode pair to effect movement of the particulates to a locationwhere the concentrated particulate suspension may be collected. In oneembodiment of the invention, the current or voltage applied to theelectrode pair is a pulsed current or pulsed voltage. In anotherembodiment, the aqueous particulate suspension is directed through aplurality of electrode pairs.

The subject invention is an electrolytic filtration apparatus and methodfor the separation of suspended particles, such as algae, from anaqueous solution, such as nitrogen and phosphorous enriched water. Theinvention is based on the filtration of an aqueous particulatesuspension under the influence of an electric field. The electric fieldcomprises a current or voltage controlled field to effect movement ofthe particulates away from the filter medium, whereby clogging of thefilter medium is minimized or eliminated. In one embodiment of theinvention, the current or voltage applied to an electrode pair is apulsed current or pulsed voltage. In another embodiment, the electricfield is generated through a plurality of electrode pairs.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic representation of an aqueous particulatesuspension in the electrolytic apparatus prior to application of theelectrolytic field.

FIG. 2a is a schematic representation of a particulate movement schemein the electrolytic apparatus after application of the electrolyticfield.

FIG. 2b is a schematic representation of another particulate movementscheme in the electrolytic apparatus after application of theelectrolytic field.

FIG. 2c is a schematic representation of still another particulatemovement scheme in the electrolytic apparatus after application of theelectrolytic field.

FIG. 3 is a schematic representation of particulate movement in anelectrolytic apparatus with a filter and collection vessel for thedepleted particulate suspension and resultant solution.

FIG. 4a is a schematic representation of particulate movement in anelectrolytic apparatus with a filter and collection vessel for theconcentrated particulate suspension.

FIG. 4b is a schematic representation of particulate movement in anelectrolytic apparatus with a moving filter for collection of theconcentrated particulate suspension.

FIG. 5 is a schematic representation of algae movement in anelectrolytic apparatus with a plurality of electrode pairs.

FIG. 6 is a schematic representation of particulate movement in anelectrolytic apparatus with a filter, depicting electrodes in oneposition relative to the filter.

FIG. 7 is a schematic representation of particulate movement in anelectrolytic apparatus with a filter, depicting electrodes in anotherposition relative to the filter.

FIG. 8 is a schematic representation of particulate movement in anelectrolytic apparatus with a filter, depicting electrodes in stillanother position relative to the filter.

FIG. 9 is a schematic representation of particulate movement in anelectrolytic apparatus with a filter, depicting electrodes in stillanother position relative to the filter.

FIG. 10a is a schematic representation of a unidirectional electrolyticpulse process.

FIG. 10b is a schematic representation of a bidirectional electrolyticpulse reverse process.

The descriptions and identification of the items in the figures aretabulated in the following table.

Numeral Item Description  10 Electrolytic Apparatus  20 Multi-electrodeElectrolytic Apparatus 100 Algal Suspension/Aqueous ParticulateSuspension 120 Depleted Algal Suspension/Aqueous Particulate Suspension150 Particle 180 Concentrated Algal Suspension/Aqueous ParticulateSuspension 200 Power Supply 220 Electrical Lead 300 Anode 350 Cathode400 Filter 400a Moving Filter Belt 420 Depleted Suspension CollectionVessel 480 Concentrated Suspension Collection Vessel

DETAILED DESCRIPTION OF THE INVENTION

There are many species of algae with different characteristics thataffect their applicability to biofuel production. For example, differentalgae species have different sizes, different densities, differenthydrocarbon or energy content, different growth requirements (light,food, media, etc.), and different predominant extracellular andintracellular hydrocarbons. In many respects, algae have thecharacteristics of particles. In one respect, particles in an aqueoussuspension develop a solid-liquid interface. This solid-liquid interfaceleads to an electrochemical double layer. The first layer is called theStern layer or fixed layer and consists of a fixed charge attached tothe particle surface. The second layer is called the Gouy layer ordiffuse layer and consists of an excess of ions of opposite charge tothose of the fixed layer and a depletion of co-ions of the same chargeas those of the fixed layer. Neither of these charged layers can bemeasured directly. The interactions of the algae cells with the waterand the resulting electric repulsion between the like charged algaecells contribute to the stability of an algae suspension. Similarinteractions of the cell surfaces of other microorganisms, includingyeasts, bacteria, archaebacteria, and the like, as well as somehydrocolloids, can contribute to the stability of respectivesuspensions, and thus, although this application refers to theprocessing of algal suspensions as a preferred use, one skilled in theart will recognize that the algal suspensions discussed herein arespecific examples of a more general aqueous particulate suspension, andthat the discussion hereafter may be readily generalized for applicationto such aqueous particulate suspensions.

The intensity of the charge on the algae cell is dependent on a numberof factors, including, 1) algae species, 2) dissolved solution ions andsolution constituents, and 3) solution pH. Therefore, various algaespecies and the environments in which they reside are characterized by azeta potential, ζ. The zeta potential is the potential differencebetween the bulk solution and the stationary layer of solution attachedto the subject particle. The zeta potential cannot be measured directlybut rather is calculated using theoretical models and experimentallydetermined mobility under the influence of an electric field. Algaesurfaces are generally thought to carry a negative charge. See G. Shelefet al., “Microalgae harvesting and Processing: A Literature Review,”U.S. DOE Solar Energy Research InstituteA Substract Report (Contract No.DE-AC02-83CH10093), August 1984. Whether the algae surfaces arenegatively or positively charged, the subject invention, while not boundby any theory, creates a concentrated zone of suspended algae and adepleted zone of suspended algae by electrophoretic movement under theinfluence of an electric field or pulsed electric field. In addition, oralternately, the subject invention may disrupt the algal suspension byweakening the repulsive affect of adjacent Gouy layers, allowing forcontact between respective particle surfaces and, at least in the caseof suspended organic materials such as algae, agglomeration of theparticulates into larger conglomerates. Consequently, the conglomeratesmay float or settle within the solution due to bulk density differencesbetween the conglomerate and the solution caused by the applied electricfield(s).

Conventional electrophoretic processing is accomplished using constantvoltage or current conditions for the duration of the process. Duringelectrophoretic processing under constant voltage control, the driftvelocity of spherical particles in suspension can be expressed asV=⅔ε₀ε_(r)ζη⁻¹(df/dx)  (1)where ε₀ is the permittivity of vacuum, ε_(r) is the relativepermittivity of the solvent, ζ is the zeta potential of the particle, ηis the viscosity of the solvent, and df/dx represents the strength ofthe applied electric field. The drift velocity is likely to affect theprocessing rate as higher drift velocities will yield greater algaemovement, and therefore faster concentration.

The equation suggests that the drift velocity of the spherical particles(in this case, algae) in the suspension is a function of the appliedelectric field during the electrophoretic process. This implies that thehomogeneity of the electric field (i.e., the “current distribution”) isimportant for a uniform, energy efficient electrolytic concentrationprocess through controlling the distribution of the concentrated algaemass.

FIG. 1 is a schematic representation of an algal suspension (100) in theelectrolytic apparatus (10) prior to application of the electrolyticprocess. The algal suspension (100) contains suspended algae particles(150) and is collected from an algae pond, a bioreactor, or a hybridcovered pond. The electrolytic apparatus (10) contains an anode (300)and a cathode (350) in electrical contact to a power supply (200) withelectrical leads (220). The gap “d” between the anode (300) and cathode(350) is determined by one skilled in the art as trade-off of operatingcost considerations and capital cost considerations. The gap can begenerally as large as 5 m or generally as small as 0.5 cm. The anode(300) and cathode (350) should exhibit good stability in theelectrolytic concentration environment. In one example, both the anodeand the cathode consist essentially of a titanium substrate with a mixedoxide coating.

FIG. 2 represents three different electrolytic algae concentrationpaths. Depending on the algae species, the nature of the aqueoussolution (such as salinity and/or specific gravity), and theelectrolytic process parameters applied, the algae could move toward oneelectrode or the other, float on the top of the solution, or settle tothe bottom of the solution. Movement in general and flotation versussettling will be influenced by the density/lipid content of the specificalgae species. FIG. 2a is a schematic representation of an electrolyticconcentration scheme in the electrolytic apparatus (10) afterapplication of the electrolytic field wherein concentrated algalsuspension (180) is formed proximate the cathode (350) leaving depletedalgal suspension (120) generally between the anode (300) and cathode(350). FIG. 2b is a schematic representation of another electrolyticconcentration scheme in the electrolytic apparatus (10) afterapplication of the electrolytic field wherein concentrated algalsuspension (180) is formed proximate the anode (300) leaving depletedalgal suspension (120) generally between the anode (300) and cathode(350). FIG. 2c is a schematic representation of still anotherelectrolytic concentration scheme in the electrolytic apparatus (10)after application of the electrolytic field wherein concentrated algalsuspension (180) floats toward the surface of the electrolytic apparatus(10) leaving depleted algal suspension (120) generally between the anode(300) and cathode (350).

FIG. 3 is a schematic representation of an electrolytic apparatus (10)containing a filter (400) positioned below a vertically oriented anode(300) and a vertically oriented cathode (350), and above a depletedsuspension collection vessel (420). After application of theelectrolytic field, concentrated algal suspension (180) floats towardthe surface of the electrolytic apparatus (10) and away from the filter(400) leaving depleted algal solution (120) adjacent to said filter(400). The depleted algal solution (120) freely flows due to the forceof gravity through the filter (400) and into the depleted suspensioncollection vessel (420). One skilled in the art will recognize that theflow through the filter (400) could be assisted by pressurization of asubstantially enclosed electrolytic apparatus (10), vacuum suctionapplied to a substantially enclosed depleted suspension vessel (420), orthe like. In this manner, clogging of the filter (400) with algaeparticles (150) is minimized or potentially eliminated, especiallycompared to flowing concentrated algal solution (180) through the filter(400).

FIG. 4a is a schematic representation of an electrolytic apparatus (10)containing a filter (400) positioned below a vertically oriented anode(300) and a vertically oriented cathode (350), and an associatedconcentrated suspension collection vessel (480). After application ofthe electrolytic field, the concentrated algal suspension (180) floatstoward the surface of the electrolytic apparatus (10) and away from thefilter (400). The concentrated algal suspension (180) is collected fromthe surface of the electrolytic apparatus and transferred to theconcentrated suspension collection vessel (480). The collection of theconcentrated algal solution (120) may be accomplished by any number ofmeans known to those skilled in the art, including, but not limited to,skimming, overflow, suction, or the like. The depleted algal solution(120) may freely flow due to the force of gravity through the filter(400) for reuse, reprocessing into a suitable aqueous growth medium, orfurther treatment and disposal.

FIG. 4b is a schematic representation of an electrolytic apparatus (10)containing a moving filter belt (400 a) positioned below a verticallyoriented anode (300) and a vertically oriented cathode (350). Afterapplication of the electrolytic field, the concentrated algal suspension(180) floats toward the surface of the electrolytic apparatus (10) andaway from the moving filter belt (400 a). The concentrated algalsuspension (180) may be collected from the surface of the electrolyticapparatus and transferred onto the moving filter belt (400 a). Aftertransfer of the concentrated algal suspension (180) onto the movingfilter belt (400 a), the material may be further concentrated by dryingor applying heat, and the dried material may be subsequently removedfrom the belt. The removal may be accomplished by any number of meansknown to those skilled in the art, including, but not limited to,scraping or the like. The moving filter belt (400 a), after removal ofthe dried material, is then available for reuse in filtering additionaldepleted algal solution (102) and handling concentrated algal solution(180). While the figure illustrates a particular collection scheme foruse with the moving filter belt (400 a) one skilled in the art willrecognize that various collection schemes, including, but not limitedto, skimming, overflow, suction, or the like, may transfer theconcentrated algal suspension (180) to the moving filter belt (400 a).

FIG. 5 is a schematic representation of a multi-electrode electrolyticapparatus (20) containing a filter (400) positioned below a plurality ofvertically oriented cathodes (350) and vertically oriented anodes (300).After application of the electrolytic field, concentrated algalsuspension (180) floats toward the surface of the multielectrodeelectrolytic apparatus (20) and away from the filter (400) leavingdepleted algal suspension (120) adjacent to said filter (400). Thedepleted algal suspension (120) freely flows due to the force of gravitythrough the filter (400) and into the depleted suspension collectionvessel (420). One skilled in the art will recognize that the flowthrough the filter (400) could be assisted by pressurization of themulti-electrode electrolytic apparatus (20) above the filter (400),vacuum suction applied to the multielectrode electrolytic apparatus (20)below the filter (400), or the like. In this manner, clogging of thefilter (400) with algae particles (150) is minimized or potentiallyeliminated, especially compared to flowing concentrated algal solution(180) through the filter (400).

FIG. 6 is a schematic representation of an electrolytic apparatus (10)wherein the filter (400) is positioned between an anode (300) and acathode (350). While the illustration depicts the anode (300) positionedbelow the filter (400) and the cathode (350) positioned above the filter(400), one skilled in the art will recognize that the anode (300) couldbe positioned above the filter (400) and the cathode (350) could bepositioned below the filter (400), as appropriate. After application ofthe electrolytic field, concentrated algal suspension (180) floatstoward the surface of the electrolytic apparatus (10) and away from thefilter (400) leaving depleted algal suspension (120) adjacent to saidfilter (400). The depleted algal suspension (120) freely flows due tothe force of gravity through the filter (400) and into the depletedsuspension collection vessel (420). One skilled in the art willrecognize that the flow through the filter (400) could be assisted bypressurization of the electrolytic apparatus (10) above the filter(400), vacuum suction applied to the electrolytic apparatus (10) belowthe filter (400), or the like. In this manner, clogging of the filter(400) with algae particles (150) is minimized or potentially eliminated,especially compared to flowing concentrated algal solution (180) throughthe filter (400).

FIG. 7 is a schematic representation of an electrolytic apparatus (10)wherein the filter (400) is positioned below a horizontally orientedanode (300) and a horizontally oriented cathode (350). While theillustration depicts the anode (300) positioned below the cathode (350),one skilled in the art will recognize that the anode (300) could bepositioned above the cathode (350), as appropriate. After application ofthe electrolytic field, concentrated algal suspension (180) floatstoward the surface of the electrolytic apparatus (10) and away from thefilter (400), leaving depleted algal suspension (120) adjacent to saidfilter (400). The depleted algal suspension (120) freely flows due tothe force of gravity through the filter (400) and into the depletedsuspension collection vessel (420). One skilled in the art willrecognize that the flow through the filter (400) could be assisted bypressurization of the electrolytic apparatus (10) above the filter(400), vacuum suction applied to the electrolytic apparatus (10) belowthe filter (400), or the like. In this manner, clogging of the filter(400) with algae particles (150) is minimized or potentially eliminated,especially compared to flowing concentrated algal solution (180) throughthe filter (400).

FIG. 8 is a schematic representation of an electrolytic apparatus (10)wherein the filter (400) is positioned below a horizontally orientedanode (300) and a horizontally oriented cathode (350). While theillustration depicts the cathode (350) positioned near the surface ofthe electrolytic apparatus (10), one skilled in the art will recognizethat the anode (300) could be positioned near the surface of theelectrolytic apparatus (10), as appropriate. After application of theelectrolytic field, concentrated algal suspension (180) floats towardthe surface of the electrolytic apparatus (10) and away from the filter(400), leaving depleted algal suspension (120) adjacent to said filter(400). The depleted algal suspension (120) freely flows due to the forceof gravity through the filter (400) and into the depleted suspensioncollection vessel (420). One skilled in the art will recognize that theflow through the filter (400) could be assisted by pressurization of theelectrolytic apparatus (10) above the filter (400), vacuum suctionapplied to the electrolytic apparatus (10) below the filter (400), orthe like. In this manner, clogging of the filter (400) with algaeparticles (150) is minimized or potentially eliminated, especiallycompared to flowing concentrated algal solution (180) through the filter(400).

FIG. 9 is a schematic representation of an electrolytic apparatus (10)wherein the filter (400) is positioned above a horizontally orientedanode (300) and below a horizontally oriented cathode (350). While theillustration depicts the anode (300) positioned below the filter (400)and the cathode (350) positioned above the filter (400), one skilled inthe art will recognize that the anode (300) could be positioned abovethe filter (400) and the cathode (350) positioned below the filter(400), as appropriate. After application of the electrolytic field,concentrated algal suspension (180) floats toward the surface of theelectrolytic apparatus (10) and away from the filter (400), leavingdepleted algal suspension (120) adjacent to said filter (400). Thedepleted algal solution (120) freely flows due to the force of gravitythrough the filter (400) and into the depleted suspension collectionvessel (420). One skilled in the art will recognize that the flowthrough the filter (400) could be assisted by pressurization of theelectrolytic apparatus (10) above the filter (400), vacuum suctionapplied to the electrolytic apparatus (10) below the filter (400), orthe like. In this manner, clogging of the filter (400) with algaeparticles (150) is minimized or potentially eliminated, especiallycompared to flowing concentrated algal solution (180) through the filter(400).

Generally, electrolytic processes conducted in aqueous based solutionwill electrolyze water to oxygen and hydrogen gas. The reaction at theanode at low pH under generally acidic conditions is:H₂O→½O₂+2H⁺+2e ⁻The reaction at the anode at high pH under generally alkaline conditionsis:2OH⁻→½O₂+H₂O+2e ⁻The reaction at the cathode at low pH under generally acidic conditionsis:2H⁺+2e ⁻→H₂The reaction at the cathode at high pH under generally alkalineconditions is:2H₂O+2e ⁻→H₂+2OH⁻Under either low or high pH conditions, the net reaction is waterelectrolysis:H₂O→½O₂+H₂The water electrolysis reactions require significant energy andtherefore it is desirable to minimize water electrolysis in order tominimize energy consumption associated with the electrolytic process. Anapproach to minimize water electrolysis is through the use of highovervoltage anodes (300) and cathodes (350) with electrically conductivediamond coatings as described previously in U.S. Pat. No. 5,900,127issued May 4, 1999 to Iida. Another approach to minimize waterelectrolysis is to increase the gap between the anode (300) and cathode(350) so that the voltage drop across the electrolyte is sufficient todecrease the differential voltage near an individual electrode whilestill effecting the movement of the suspended algae.

In the instant invention, pulse and pulse reverse electrolytic fieldsare used to induce movement of suspended particles, such as algae, in asolution. Pulse and pulse reverse electrolytic fields offer benefitssuch as reduced electrolysis and therefore reduced energy consumption. Ageneralized pulse waveform with unidirectional pulses is shown in FIG.10a . Although the generalized pulse waveform is depicted for anodicpulses at the anode (300), one skilled in the art would recognize thatthe unidirectional pulses are cathodic at the cathode (350). Inaddition, FIG. 10a only depicts two unidirectional pulse cycles. Theperiod T of the waveform is the sum of the anodic on-time t₁ andrelaxation period t₀ (T=t₁+t₀). The inverse (1/T) of the period T of thewaveform is the frequency f of the waveform. The frequency of theunidirectional pulse waveform is typically 10 to 1000 Hz and moretypically 50 to 500 Hz. The ratio (t₁/T) of the anodic on-time t₁ to theperiod T is the anodic duty cycle D₁. The duty cycle of theunidirectional pulse waveform is typically 10 to 80% and more typically20 to 60%. The unidirectional pulses are either under voltage control orcurrent control. The voltage during the anodic on-time t₁ is referred toas the anodic peak pulse voltage (V₁). The voltage of the unidirectionalpulse waveform is typically 20 to 400V DC, and more typically 50V DC.The current during the anodic on-time t₁ is referred to as the anodicpeak pulse current (I₁). The anodic charge Q₁ is the product of theanodic current density I₁ and time t₁ (Q₁=I₁t₁). The average currentI_(ave) is the average anodic current (I_(ave)=D₁I₁).

A generalized pulse waveform with bidirectional pulses is shown in FIG.10b . Although the generalized bidirectional pulse waveform is depictedfor anodic pulses and cathodic pulses at the anode (300), one skilled inthe art would recognize that the bidirectional pulses are cathodicpulses and anodic pulses at the cathode (350). FIG. 10b only depicts twobidirectional pulse cycles. The period T of the waveform is the sum ofthe anodic on-time t₁, cathodic on-time t₂, relaxation period t_(o), andintermediate period t_(i) (T=t₁+t₂+t_(o)+t_(i)). The inverse (1/T) ofthe period T of the waveform is the frequency f of the waveform. Thefrequency of the bidirectional pulse waveform is typically 10 to 1000 Hzand more typically 50 to 500 Hz. The ratio (t₁/T) of the anodic on-timet₁ to the period T is the anodic duty cycle D₁ and the ratio (t₂/T) ofthe cathodic on-time t₂ to the period T is the cathodic duty cycle D₂.The duty cycle of the bidirectional pulse waveform is typically 10 to80% and more typically 20 to 60%. The bidirectional pulses are eitherunder voltage control or current control. The voltage during the anodicon-time t₁ is referred to as the anodic peak voltage (V₁) and thevoltage during the cathodic on-time t₂ is referred to as the cathodicpeak voltage (V₂). The peak voltage of the bidirectional pulse waveformis typically 20 to 400V DC. The current during the anodic on-time t₁ isreferred to as the anodic peak current (I₁) and the current during thecathodic on-time t₂ is referred to as the cathodic peak current (I₂).The anodic charge Q₁ is the product of the anodic current density I₁ andthe anodic on-time t₁ (Q₁=I₁t₁), while the cathodic charge Q₂ is theproduct of the cathodic current density I₂ and the cathodic on-time t₂(Q₂=I₂t₂). The average current density (I_(ave)) is the average anodiccurrent density (D₁I₁) minus the average cathodic current density(D₂I₂).

EXAMPLES

Electrolytic concentration experiments were conducted with aqueoussuspensions of the algae species Scenedesmus dimorphous cultured in acovered freshwater pond. The concentration of the S. dimorphous prior toand after application of the electrolytic field was determined using ahemacytometer used for cell counts. The initial concentration wasdetermined as 32,750,000 algae cells per milliliter of solution. Afterelectrolytic concentration the hemacytometer was used to determine thecell count, which was then normalized to the initial cell count. Anapparatus similar to that shown in FIG. 1 with vertically suspendedelectrodes was used for the electrolytic concentration experiments.Electrodes characterized with high overvoltages and good stability forthe water electrolysis reaction are also desirable to reduce the energyconsumption of the electrolytic process. The electrodes used in theseexperiments consisted of a titanium mesh substrate with a mixed oxidecoating, known as a dimensionally stable anode, and were obtained fromDe Nora Tech of Chadron, Ohio. The gap between the electrodes wasapproximately 2 cm.

Example 1

The applied voltage was 50V DC with an initial current of 1,300 mA and afinal current of 3,000 mA. The voltage was applied for 10 minutes.During the experiment, vigorous electrolysis was observed and the algaecollected as a floated mass at the top of the solution. Due to thevigorous electrolysis, the floated algae mass could not be collected andaccurately measured for cell count. During the course of the experiment,the container became very hot. The region between the electrodes wasvisually very clear and cell count indicated that this region wasdepleted by 181.9 times compared to the initial algae concentration. Theenergy consumption per volume was calculated as 204 W·hr/L of solution.The water consumption was calculated as 45.5 μL water/L solution (4.56μL water/(L solution·min)).

Example 2

The applied voltage was 50V DC with an initial current of 1,300 mA and afinal current of 2,000 mA. The voltage was applied for 2 minutes. Duringthe experiment, vigorous electrolysis was observed and the algaecollected as a floated mass at the top of the solution. Due to thevigorous electrolysis, the floated algae mass could not be collected andaccurately measured for cell count. The region between the electrodeswas slightly green and cell count indicated that this region wasdepleted by 7.8 times compared to the initial algae concentration. Theenergy consumption per volume was calculated as 28 W·hr/L of solution.The water consumption was calculated as 31.8 μL water/L solution (15.9μL water/(L solution·min)).

Example 3

The applied unidirectional pulsed voltage was 50V PC with a duty cycleof 20% and a frequency of 100 Hz. The initial current was 1,220 mA and afinal current of 1,400 mA. The voltage was applied for 4 minutes. Duringthe experiment, minor electrolysis was observed and the algae collectedas a floated mass at the top of the solution. The floated algae wascollected and the cell count indicated the algae was concentrated by31.5 times compared to the initial algae concentration. The regionbetween the electrodes was slightly green and cell count indicated thatthis region was depleted by 12.8 times compared to the initial algaeconcentration. The energy consumption per volume was calculated as 0.8W·hr/L of solution. The water consumption was calculated as 0.16 μLwater/L solution (0.04 μL water/(L solution·min)).

Although various aspects of the disclosed electrolytic concentrationmethod and apparatus for the concentration of suspended algae from asolution have been shown and described, modifications may occur to thoseskilled in the art upon reading the specification.

What is claimed is:
 1. An apparatus for the concentration of suspendedparticles in an aqueous solution comprising: a) an electrolytic cellcontaining (i) at least an anode and a cathode and (ii) a moving filterbelt, the electrolytic cell receiving a solution containing suspendedalgae particles therein; b) a power supply near the filter; and c) azone of depleted suspended algae particles near the filter formed underthe influence of an applied electric field from said power supply. 2.The apparatus of claim 1, wherein said anode is positioned on one sideof said filter and said cathode is positioned on the other side of saidfilter.
 3. The apparatus of claim 1, wherein said anode is positionedabove said filter and said cathode is positioned above said anode. 4.The apparatus of claim 1, wherein said cathode is positioned above saidfilter and said anode is positioned above said cathode.
 5. The apparatusof claim 1, wherein said anode is positioned above said filter and saidcathode is positioned near the surface of said electrolytic cell.
 6. Theapparatus of claim 1, wherein said cathode is positioned above saidfilter and said anode is positioned near the surface of saidelectrolytic cell.
 7. The apparatus of claim 1, wherein said anode ispositioned below said filter and said cathode is positioned near thesurface of said electrolytic cell.
 8. The apparatus of claim 1, whereinsaid cathode is positioned below said filter and said anode ispositioned near the surface of said electrolytic cell.
 9. The apparatusof claim 1, wherein said power supply is a pulse power supply.
 10. Theapparatus of claim 1, wherein said anode and said cathode are highovervoltage electrodes.
 11. The apparatus of claim 10, wherein saidpower supply is adapted to apply an electric field comprising a voltagedifference across said anode and said cathode of about 20 to 400 V DC.12. The apparatus of claim 1, wherein said electrolytic cell comprises aplurality of said anodes and said cathodes.
 13. The apparatus of claim1, further comprising a depleted suspension collection vessel to receivedepleted suspended algae particles.
 14. The apparatus of claim 1,further comprising a concentrated suspension collection vessel toreceive concentrated suspended algae particles.
 15. The apparatus ofclaim 1, wherein the moving filter belt is adapted to transport aconcentrated algal suspension away from the electrolytic cell.